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Journal of the Physical Society of Japan

Mode Analysis of Fluctuations in Two-Species Bose–Einstein Condensates of Atomic Gases with a Vortex for a Component –Characteristic Features of Compressive and Sliding Motions–

Kensuke Doi, Yuhei Natsume

Published Apr 15, 2003• 6 citations
Publisher

Abstract

Characteristic features of fluctuations of Bose–Einstein condensations for systems of two-components in gas phases of alkali-metal atoms trapped by spherical harmonic potentials are discussed on the basis of numerical calculations. We concentrate our attention on the phases in which the spherical state ψ 1 without vortex is surrounded by ψ 2 with a vortex for the unit circulation q =1. These states are expressed by Gross–Pitaevskii equation, where a vortex-core is along the z -axis. We investigate properties of collective excitations by the linear analysis for bosonic excitations described as Bogoliubov equations. The behavior of each mode is discussed in relation with the role of interspecies repulsion in addition to that of intraspecies one. We point out the role of the new compressive mode which has two nodes on z -axis, in addition to that of the core mode without a node which have been previously discussed in the single-component system. Furthermore, we would like to emphasize that sliding modes show the branching features into in- and out-of-phase motions with increasing interspecies interaction. The dependence of those branchings on interspecies repulsion is explained by spatial shapes of relevant modes.

DOI
10.1143/jpsj.72.835

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Characteristic features of fluctuations of Bose–Einstein condensations for systems of two-components in gas phases of alkali-metal atoms trapped by spherical harmonic potentials are discussed on the basis of numerical calculations. We concentrate our attention on the phases in which the spherical state ψ 1 without vortex is surrounded by ψ 2 with a vortex for the unit circulation q =1. These states are expressed by Gross–Pitaevskii equation, where a vortex-core is along the z -axis. We investigate properties of collective excitations by the linear analysis for bosonic excitations described as Bogoliubov equations. The behavior of each mode is discussed in relation with the role of interspecies repulsion in addition to that of intraspecies one. We point out the role of the new compressive mode which has two nodes on z -axis, in addition to that of the core mode without a node which have been previously discussed in the single-component system. Furthermore, we would like to emphasize that sliding modes show the branching features into in- and out-of-phase motions with increasing interspecies interaction. The dependence of those branchings on interspecies repulsion is explained by spatial shapes of relevant modes.
Characteristic features in phases of Bose-Einstein condensations (BEC) for systems of two-components in gas phases of Alkali-metal atoms trapped by harmonic potentials are discussed on the basis of numerical calculations by the use of Gross-Pitaevskii equation. Condensations with and without vortices are considered in order to make clear spatial distributions. The shape of spatial distribution is essentially dependent on the ratio of interaction strengths between intra-components and inter-components. We emphasize the possibility of coexistence of vortex-free state (where the angular momentum ℓ is 0.) for the one component in BEC and vortex state (ℓ z =1) for another component whose shape is the donut-like spatial distribution. Furthermore, fluctuations of these states are discussed. In partiucular, we concentrate our attention to the appearance of the core modes with ℓ z =0 symmetry for the component with a vortex. In connection with the behavior of the core modes, the contribution of the component without a vortex to the stability of the above vortex is discussed on the basis of numerical calculation.
We examine the dynamics associated with the miscibility-immiscibility transition of trapped two-species Bose-Einstein condensates (TBECs) of dilute atomic gases in the presence of vortices. In particular, we consider TBECs of Rb hyperfine states and Rb-Cs mixture. In the case of a singly charged vortex in only one of the condensates, there is enhancement when the vortex is present in the species which occupies the edges at phase separation. But suppression occurs when the vortex is in the species which occupies the core region. To examine the role of the vortex, we quench the interspecies interactions to drive the TBECs from miscible to immiscible phase, and use the time-dependent Gross-Pitaevskii equation to probe the phenomenon of phase separation. We also examine the effects of higher charged vortex to phase separation.
Summary form only given. I present results of the detailed asymptotic and numerical results on stability of unit and higher order vortices in coupled Gross-Pitaevskii equations, which model the experimental work. Low lying eigenfrequencies, including ones generating instability of the unit vortex placed in one component of a binary mixture of BEC, have been calculated applying perturbative theory in the limit of the weak interparticle interaction. Results of the perturbative theory have been verified and significantly extended using numerical analysis of the linearised equations for excitations and direct numerical simulation of the coupled Gross-Pitaevskii equations. Two main instability mechanisms have been identified and their dependencies on the relative number of the particles in the condensates and on the strength of the inter-species interaction have been studied. The first mechanism is the drift instability induced by the vortex free component of the condensate and resulting in the motion of the vortex out of the condensate center to its periphery. The second mechanism is the splitting of the higher order vortices into unit ones. Absolute and/or relative increase of the number of particles in the vortex free condensate component has been found to have stabilizing effect on both drift and splitting instabilities. Different scenarios of the splitting instabilities have been found.
This dissertation studies the dynamics of atomic Bose-Einstein condensates (nEes) and Bose gases in a suddenly modified potential.\nFirstly, we investigate the correlation between vortex formation and interference in merging Bose-Einstein condensates. This inherent correlation can explain some experiments in which vortices are formed in interfering condensates. Furthermore, we show the interference properties of merging condensates, particularly the relation of interference among colliding, expanding, and merging condensates, which can explain some complex interference phenomena in recent experiments.\nSecondly, using the truncated Wigner approximation, we investigate the role of quantum fluctuations in different forms on the transport properties of bosonic atoms in a ID optical lattice. The dynamics of transport with respect to quantum fluctuations in the plane-wave modes is distinct from that in the single-harmonic-oscillator modes. The discrepancies are demonstrated in detail. Quantum fluctuations in Bogoliubov modes lead to stronger damping behaviour of the centre-of-mass motion than quantum fluctuations in the plane-wave and single-harmonic-oscillator modes, which is in agreement with the experiment.\nThirdly, the role of the relative phase variation and velocity of two low-density condensates, and quantum noise on interference properties are discussed. In particular, the incoherent atoms have significant effect on the interference visibility and microscopic dynamics. Although the interference pattern is not broken by quantum fluctuations, indicating the robust character of this interference, the process of inner correlations and dynamics is very complex and cannot he understood purely with mean-field theory.\nFinally, we investigate the elementary excitation spectrum and mode functions of a trapped Bose gas by numerically solving the Bogoliubov-De Gennes equation. The characteristic form of the Bogoliubov matrix, determined by the interatomic interactions, and the interaction between atoms and confining potential, specifies excitation spectra and mode functions. The role of these interactions on the properties of spectra and mode functions are shown.
The linearized time-dependent coupled Gross-Pitaevskii equations describing the long-wavelength excitations of Bose-Einstein condensed binary mixtures are solved in harmonic traps for the case where only the binary phase is present. The dispersion law contains two branches whose dependence on the quantum numbers of the modes is the same as for a one-component condensate, but with different prefactors, depending on the ratios of the three sss-wave scattering lengths of the two atomic species. In the general case where the binary phase in the trap coexists with one or both one-component phases, the mode spectrum depends on the geometry of the interphase boundaries due to the boundary conditions there. Measurements of the oscillation frequencies as in recent experiments with modulated traps would yield very detailed information on this system.
In this work, we consider vortex lattices in rotating Bose–Einstein condensates composed of two species of bosons having different masses. Previously (Barnett et al 2008 New J. Phys. 10 043030), it was claimed that the vortices of the two species form bound pairs and the two vortex lattices lock. Remarkably, the two condensates and the external drive all rotate at different speeds owing to the disparity of the masses of the constituent bosons. In this paper, we study the system by solving the full two-component Gross–Pitaevskii equations numerically. Using this approach, we verify the stability of the putative locked state that is found to exist within a disc centered on the axis of rotation and that depends on the mass ratio of the two bosons. We also derive a refined estimate for the locking radius tailored to the experimentally relevant case of a harmonic trap and show that this agrees with the numerical results. Finally, we analyze in detail the rotation rates of the different components in the locked and unlocked regimes.